op_insert ('a -> 'a -> bool) -> 'a -> 'a list -> 'a list
Add an element to a list if it is not already there.
If there exists an element y in list, such that eq x y, then insert eq x list equals list. Otherwise, x is added to list.
Never fails.
- op_insert (fn x => fn y => x = y mod 2) 1 [3,2];
> val it = [3, 2] : int list

- op_insert aconv (Term `\x. x /\ y`)
                  [T, Term `\z. z /\ y`, F];
> val it = [`T`, `\z. z /\ y`, `F`] : term list

- op_insert aconv (Term `\x. x /\ y`)
                  [T, Term `\z. z /\ a`, F];
> val it = [`\x. x /\ y`, `T`, `\z. z /\ a`, `F`] : term list

There is no requirement that eq be recognizable as a kind of equality (it could be implemented by an order relation, for example).

One should not write code that depends on the arrangement of elements in the result.

A high-performance implementation of finite sets may be found in structure HOLset.

HOL  Kananaskis-14